OK, serial can be found in a similar way to a standard radio/cd unit (hold preset 1 & 6 down while powering on).
Edit: Just watched a demo video of the SD Navi unit, different buttons to the generic version below but serial may be easier to get if it lets you do this - press Menu, then select Information, then Navigation Version, serial should be on screen. The method below makes less sense for SD units but maybe the only option when code locked.
For some Honda Navi units generic instructions suggests hold down Map, Menu & Cancel (or Back depending on labels) for a good 10 seconds at the code entry screen. From the menu pick Self Diagnosis Mode (or Unit Check on other models), then ECU info (or Navi ECU). Let it do it's thing and the code should be shown when finished. May have 3 letters in front of the 8 digits, but I probably only need the 8 digits like 23001234.
Your code might already be on a sticker in the glove box or under the fuse box cover.
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TG
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